[Linux] C++ project practice-UDP server

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UDP communication

Function analysis

#include <sys/types.h>
#include <sys/socket.h>
ssize_t sendto(int sockfd, const void *buf, size_t len, int flags, 
					const struct sockaddr *dest_addr, socklen_t addrlen);
		- 参数：
			- sockfd：通信的fd
			- buf：要发送的数据
			- len：发送数据的长度
			- flags：0
			- dest_addr：通信的另外一端的地址信息
			- addrlen：地址的内存大小
		- 返回值：
			- 失败：-1
			- 成功：发送字节的数量
ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
					struct sockaddr *src_addr, socklen_t *addrlen);
		- 参数：
			- sockfd：通信的fd
			- buf：接收数据的数组
			- len：数组的大小
			- flags：0
			- src_addr：用来保存另一端的地址信息，不需要可以指定为NULL
			- addrlen：地址的内存大小

Code example

Server side code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <arpa/inet.h>

int main() {
    
    

    // 1.创建一个通信的socket
    int fd = socket(PF_INET, SOCK_DGRAM, 0);
    
    if(fd == -1) {
    
    
        perror("socket");
        exit(-1);
    }   

    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons(9999);
    addr.sin_addr.s_addr = INADDR_ANY;

    // 2.绑定
    int ret = bind(fd, (struct sockaddr *)&addr, sizeof(addr));
    if(ret == -1) {
    
    
        perror("bind");
        exit(-1);
    }

    // 3.通信
    while(1) {
    
    
        char recvbuf[128];
        char ipbuf[16];

        struct sockaddr_in cliaddr;
        int len = sizeof(cliaddr);

        // 接收数据
        int num = recvfrom(fd, recvbuf, sizeof(recvbuf), 0, (struct sockaddr *)&cliaddr, &len);

        printf("client IP : %s, Port : %d\n", 
            inet_ntop(AF_INET, &cliaddr.sin_addr.s_addr, ipbuf, sizeof(ipbuf)),
            ntohs(cliaddr.sin_port));

        printf("client say : %s\n", recvbuf);

        // 发送数据,+是因为字符串的结束符也要发过去 
        sendto(fd, recvbuf, strlen(recvbuf) + 1, 0, (struct sockaddr *)&cliaddr, sizeof(cliaddr));

    }

    close(fd);
    return 0;
}

Client code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <arpa/inet.h>

int main() {
    
    

    // 1.创建一个通信的socket
    int fd = socket(PF_INET, SOCK_DGRAM, 0);
    
    if(fd == -1) {
    
    
        perror("socket");
        exit(-1);
    }   

    // 服务器的地址信息
    struct sockaddr_in saddr;
    saddr.sin_family = AF_INET;
    saddr.sin_port = htons(9999);
    inet_pton(AF_INET, "127.0.0.1", &saddr.sin_addr.s_addr);

    int num = 0;
    // 3.通信
    while(1) {
    
    

        // 发送数据
        char sendBuf[128];
        sprintf(sendBuf, "hello , i am client %d \n", num++);
        sendto(fd, sendBuf, strlen(sendBuf) + 1, 0, (struct sockaddr *)&saddr, sizeof(saddr));

        // 接收数据
        int num = recvfrom(fd, sendBuf, sizeof(sendBuf), 0, NULL, NULL);
        printf("server say : %s\n", sendBuf);

        sleep(1);
    }

    close(fd);
    return 0;
}

Question: Can't tell when to use strlen and when to use sizeof? The parameters passed by the recvfrom/sendto function seem to be the buffer size. Why does the latter use strlen for parameters?
Answer: sizeof calculates the size of the memory space occupied by the allocated array, which is not affected by the content stored in it; strlen calculates the length of the string, with '\0' as the end mark of the string.
Here recvfrom wants to receive a string of unknown length, so it specifies the buffer and buffer size used for reception; and sendto wants to send a string and specify the length of the sent data.

broadcast

Function analysis

int setsockopt(int sockfd, int level, int optname, const void *optval, socklen_t optlen);  //修改sock属性
			- sockfd：文件描述符
			- level：SOL_SOCKET
			- optname：SO_BROADCAST
			- optval：int类型的值，为1表示允许广播
			- optlen：optval的大小

Code example

Client code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <arpa/inet.h>

int main() {
    
    

    // 1.创建一个通信的socket
    int fd = socket(PF_INET, SOCK_DGRAM, 0);
    if(fd == -1) {
    
    
        perror("socket");
        exit(-1);
    }   

    struct in_addr in;

    // 2.客户端绑定本地的IP和端口
    struct sockaddr_in addr;
    addr.sin_family = AF_INET;
    addr.sin_port = htons(9999);
    addr.sin_addr.s_addr = INADDR_ANY;

    int ret = bind(fd, (struct sockaddr *)&addr, sizeof(addr));
    if(ret == -1) {
    
    
        perror("bind");
        exit(-1);
    }

    // 3.通信
    while(1) {
    
    
        
        char buf[128];
        // 接收数据
        int num = recvfrom(fd, buf, sizeof(buf), 0, NULL, NULL);
        printf("server say : %s\n", buf);

    }

    close(fd);
    return 0;
}

Server side code:

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <arpa/inet.h>

int main() {
    
    

    // 1.创建一个通信的socket
    int fd = socket(PF_INET, SOCK_DGRAM, 0);
    if(fd == -1) {
    
    
        perror("socket");
        exit(-1);
    }   

    // 2.设置广播属性
    int op = 1;
    setsockopt(fd, SOL_SOCKET, SO_BROADCAST, &op, sizeof(op));
    
    // 3.创建一个广播的地址
    struct sockaddr_in cliaddr;
    cliaddr.sin_family = AF_INET;
    cliaddr.sin_port = htons(9999);
    inet_pton(AF_INET, "192.168.177.255", &cliaddr.sin_addr.s_addr);

    // 3.通信
    int num = 0;
    while(1) {
    
    
       
        char sendBuf[128];
        sprintf(sendBuf, "hello, client....%d\n", num++);
        // 发送数据
        sendto(fd, sendBuf, strlen(sendBuf) + 1, 0, (struct sockaddr *)&cliaddr, sizeof(cliaddr));
        printf("广播的数据：%s\n", sendBuf);
        sleep(1);
    }

    close(fd);
    return 0;
}